1. Field of the Invention
The present invention relates to a voltage controlled oscillator in a phase-lock-loop and, in particular, to a range programmable voltage controlled oscillator in an autoranging phase-lock-loop.
2. Description of the Related Art
A voltage controlled oscillator (VCO) is a device which can generate an arbitrary number of oscillator signals over a frequency range by varying an input voltage signal. In practice, the frequency range of the VCO is limited by the required stability of the oscillator signal. A stable oscillator signal is an oscillator signal the frequency of which remains constant within the error tolerances of the circuit which is to receive the oscillator signal.
A number of factors can effect the frequency of an oscillator signal. First, the frequency of an oscillator signal can be affected by fluctuations of the input voltage signal. The effect of fluctuations of the input voltage signal presents a design trade-off between stability and frequency coverage.
For example, a high-gain VCO may operate over a frequency range of 300 MHz (megahertz) by varying the input voltage signal from one to four volts. When a voltage fluctuation of one tenth of a volt occurs, the oscillator signal may temporarily shift its frequency by as much as 10 MHz.
Whether such a shift poses design problems depends on the error tolerances of the circuit which is to receive the oscillator signal. For circuits which cannot tolerate a sudden frequency shift of 10 MHz, the shift can produce unacceptable timing errors.
A low-gain VCO, on the other hand, operates over a much smaller frequency range, thereby minimizing the effects of any fluctuations of the input voltage signal. For example, a low-gain VCO may operate over a frequency range of 75 MHz over the same input voltage range. With a low-gain VCO, each one tenth of a volt voltage fluctuation produces a much smaller frequency shift of approximately 2.5 MHz.
Thus, by reducing the frequency range, a more stable oscillator signal is generated. The problem, however, is that such a narrow frequency range severely limits the application of the low-gain VCO.
Traditionally, the VCO is utilized to track a reference signal. The VCO tracks the reference by utilizing a phase-lock-loop which provides the input voltage signal to the VCO by comparing the oscillator signal that is currently being generated with the reference signal. As long as there is a difference between the current oscillator signal and the reference signal, the phase-lock-loop continues to change the input voltage, thereby changing the current oscillator signal. When the current oscillator signal and the reference signal are matched, the phase-lock-loop stops changing the input voltage, thereby maintaining an oscillator signal which is locked to the referenced signal.
Variations in the surrounding temperature can cause the frequency of the oscillator signal to shift. Although a closed phase-lock-loop will adjust
itself for temperature, supply voltage, and process variations by adjusting itself to a new bias point, a low-gain VCO which, as in the above example, is limited to a 75 MHz frequency range, will not be able to continually track the reference signal if the oscillator signal drifts outside of the 75 MHz range. This is particularly likely to happen if the original reference signal is near the end of the frequency range.
In addition, having such a narrow frequency range requires that the reference signal be matched to the VCO. Thus, rather than having one high-gain VCO which can cover a variety of reference signals, a different low-gain VCO must be used for each reference signal which is outside of the frequency range of the low-gain VCO.
Thus, there is a need for a VCO which has multiple low-gain frequency ranges and which can generate oscillator signals with a constant frequency over variations in the surrounding temperature.